1. Field of the Invention
The present invention relates to an X-ray image detecting apparatus for detecting an X-ray image of a subject, such as a person to be examined for diagnosis or the like.
2. Description of the Related Art
Recently, X-ray image acquisition systems for taking X-ray images of subjects being examined for diagnosis using semiconductor sensors have been developed.
When compared with conventional X-ray radiographic systems employing ordinary silver halide photography, these X-ray image acquisition systems have such advantages in practical use that images can be recorded which have a very wide dynamic range corresponding to a very wide range in the amount of radiation, to which the sensor is exposed. That is, X-ray images can be obtained which are unlikely to be affected by variations in the amount of exposure of radiation; after X-rays with a very wide dynamic range are read with a detector including a photoelectric transducer and converted into an electric signal, the electric signal is processed so as to output X-ray images on recording materials such as a photosensitive material, and the like and on display units such as a CRT, and the like, as visible images. In this radiography, an X-ray grid, which removes scattered X-rays generated in subjects, are used in many cases in order to improve contrast in a radiographic image.
FIG. 1 is a sectional view of an X-ray grid and a detector used in a conventional radiographic apparatus. An X-ray image detector 1 is arranged such that a plurality of photoelectric conversion elements 3 are two-dimensionally disposed on an insulation substrate 2, and further, fluorescent substance 4 is laminated on the photoelectric conversion elements 3. In addition, a grid 5 is disposed above the X-ray image detector 1 with a predetermined space therebetween. The grid 5 is arranged such that foils 7, which are composed of lead or the like, having a high X-ray absorption ratio, and intermediate materials 8, which are composed of aluminum or the like, having a low X-ray absorption ratio, are held by a cover member 6. Using the grid 5 arranged as described above permits primary X-rays L1, which have passed through a subject without being scattered thereby, to pass through the grid 5 and to reach the fluorescent substance 4 of the X-ray image detector 1. When X-rays L1 are irradiated onto the fluorescent substance 4, the optical materials (light emitting materials) in the fluorescent substance 4 are excited and emit fluorescence L2 having a wavelength within the spectral sensitivity wavelength range of the photoelectric conversion elements 3. Further, X-rays which are incident on the grid 5 with a large angle with respect to the primary X-rays L1, such as a scattered X-ray component L3 generated by the subject, are absorbed by the foils 7.
During exposure of radiation, the grid 5 is moved in a direction B or C by a drive unit (not shown). With this operation, an excellent image can be obtained by the X-ray image detector 1 which has no image component of stripes of the grid 5 as well as no moires or aliasing caused by a difference between the pitch of the foils 7 and the pitch of the pixels of the X-ray image detector 1.
Radiography is required to satisfy contradictory conditions (1) that an excellent image with a high contrast is to be obtained while (2) reducing the dosage of the subjects as much as possible by reducing the amount of X-rays with which they are irradiated. However, the grid 5 shown in FIG. 1 may act as a factor for deteriorating an image by reducing the intensity of X-rays on the X-ray image detector 1.
One reason for this reducing of the intensity of X-rays is that the X-rays L1, which reach the X-ray image detector 1, must pass through the intermediate materials 8. While the intermediate materials 8 are composed of aluminum or the like having a high X-ray transmission ratio as described above, that transmittance is not 100% as a matter of fact. When, for example, the thickness xcex941 of the foils 7 is set to 43 xcexcm at a time a grid density is 40 lines/cm and a grid ratio is 10:1, the intermediate materials 8 have a thickness xcex942 of 207 xcexcm (=1 cm/40xe2x88x92xcex941) and a height xcex943 of 2070 xcexcm (=xcex942xc3x9710).
When the intermediate materials 8 are composed of aluminum, the aluminum in the above case has a thickness of about 2 mm, and the primary X-rays L1 have a transmittance of about 70%. Accordingly, about 30% of the intensity of the X-rays will be lost. Further, when viewed from the direction from which the X-rays are incident, 17% (=xcex941/(xcex942+xcex941)) of the grid 5 is composed of lead through which X-rays do not pass. Accordingly, the total X-ray transmittance of the grid 5 is about 60% (0.7xc3x97(1xe2x88x920.17)) when the loss of the intermediate materials 8 is also taken into consideration, which means that the reduction of the intensity of X-rays caused by the grid 5 is large and cannot be ignored.
Further, the fluorescence L2 generated in the fluorescent substance 4 by the primary X-rays which have passed through the grid 5, radiates in various directions because the fluorescent substance 4 is formed in a continuous flat shape so as to entirely cover the photoelectric conversion elements 3. Accordingly, this fluorescence L2 reaches not only a photoelectric conversion element 3a located just below a position where it emits but also other photoelectric conversion elements, for example, 3b, and the like adjacent to the photoelectric conversion element 3a. 
Therefore, as described below, the grid 5 reduces the intensity of X-rays, while it does remove the incident scattered X-ray component L3. Further, the continuous flat-shaped fluorescent substance 4 may deteriorate the MTF (modulation transfer function) of the X-ray image detector because the fluorescence L2 generated in the fluorescent substance 4 reaches a plurality of adjacent photoelectric conversion elements. Furthermore, when the intensity of the emitting fluorescence L2 is increased by increasing the thickness of the fluorescent substance 4 to improve the intensity of signals outputted from the photoelectric conversion elements 3, the above tendency becomes stronger, and improvement of the sensitivity of X-ray image detectors may be impeded.
Accordingly, it is an object of the present invention, which was made based on the above recognition of the problem, to provide an excellent X-ray image detecting apparatus capable of obtaining a good image having a high contrast while reducing the dosage received by a subject.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.